The present invention relates to a position measuring device, in particular, a linear encoder for measuring a relative position of two objects and including a scale attachable to one of the objects, and a device for scanning the scale and attachable to the other of the objects.
Such a linear encoder can be used, for example, for measuring a relative linear position of two machine-tool parts, such as machine-tool bed and machine-tool slide. In order to insure a very precise measurement, the scale is generally embedded in a hollow housing, with the scanning device being located in the same housing.
With such very precise encoders, the selection of material is very important, because materials which have different coefficients of thermal expansion need be combined. For different reasons, only on rare occasions, materials which have the same or similar coefficients of thermal expansion can be used. Commonly, as disclosed in European Patent No. 0.118 607, when steel workpieces are to be machined, materials such as gray cast iron (machine-tool parts), aluminum (hollow-housing) and steel or glass (the scale), are combined.
European Patent No. 0.118 607 and European Patent Application No. 0.202 630 also disclose using a scale formed of a material the coefficient of thermal expansion of which is close to zero as much as possible. The two European patents disclose suitable materials for producing scales.
It is also known to connect a scale and a scale support with a possibility of their relative longitudinal displacement, by using a highly elastic intermediate layer. Therefore, the length change of the support, which is caused by bending of the support, does not affect the scale. The connection of a scale to a support by an intermediate, highly elastic layer is disclosed in German Patent No. 1,176,382.
U.S. Pat. No. 3,816,002 discloses a linear encoder the housing of which is secured on a slide of a machine-tool. The scale is secured, inside the housing, at its one end, with the other end of the scale being positioned inside the housing by means of a tensioning device. The tensioning spring device compensates for the temperature-induced length change. Due to the use of such a tensioning device, the length change of the housing does not affect the scale, and the scale length is changed according to its own coefficient of thermal expansion. However, this can result in a measurement error.
Accordingly, the connection of a scale to a support by a highly elastic layer can only then be used when a temperature-induced error is taken into account. For the determination of this influence parameter, complex error compensation calculations have to be conducted.
Accordingly, the object of the invention is to provide a linear encoder without the above-discussed drawbacks and in which the adverse temperature influence is eliminated, and the error correction calculations can be dispensed with.